1. Field of the Invention
The present invention relates generally to a crucible in an effusion cell of a molecular beam epitaxy system for use in coating a substrate.
2. Description of the Related Art
Molecular beam epitaxy (MBE) is a method for controlling the growth of single-crystal semiconductor layers on a substrate over a range of layer thicknesses from 0.1 nm through 10 .mu.m. In a solid-source molecular beam epitaxy, materials are thermically evaporated for this purpose in an ultra-high vacuum in special crucibles. The material which is released forms a molecular beam in pressure ranges of typically less than 10.sup.-6 mbar. The evaporator for such molecular beam epitaxy systems has a certain directional characteristic which is dependent upon the shape of the crucible. In order to achieve high yields in hetero layer structure components for semiconductor manufacturing, a relatively great uniformity in layer thickness during the use of molecular beam epitaxy is a prerequisite.
The uniformity of the thickness of a semiconductor layer when using a solid-source molecular beam epitaxy system is determined by:
1. the shape of the crucible,
2. the geometrical arrangement of the crucible with respect to the substrate, and
3. the rotation of the substrate.
Various materials in various effusion cells are used for the manufacture of more recent semiconductor structures. To that end, it is necessary to provide an arrangement of the effusion cells which is rotationally symmetrical to the normal of the surface of the substrate. Any asymmetry arising therefrom in the distribution of the material flow over the substrate wafer is, in turn, partially compensated by rotating the wafer.
There are two concepts or methods which have been essentially pursued up to now to optimize the uniformity of the layer thicknesses in epitaxy systems. The first concept is the employment of a cylindrical, slightly conical crucible having a small opening that produces advantages in the temperature regulation. Furthermore, such crucibles produce strong collimation of the molecular beam, and thus result in, only slight losses of material. The disadvantage in the use of such crucible with strong beam collimation is that uniformity of layer thickness is not easily achieved, although attempts to compensate for the lack of uniformity include dislocation of the maximum of the flow distribution from the center of the substrate to the edge of the substrate by tilting the axis of the cell. Using rotation of the substrate as an averaging method, a smaller layer thickness variation over a larger substrate region is achieved than without tilting.
The second concept is the use of more pronouncedly conical crucibles having a circular cross section in which the whole source surface has direct visual contact with the entire substrate surface. This means that the cross section of the crucible here as well as below is a section perpendicular to the center axis of the crucible, i.e. perpendicular to the molecular beam direction. The axis of symmetry is thereby aimed nearly at the center of the substrate, as disclosed, for example, in the publication of J. A. Curless, J.Vac.Sci.Technol.B3 (2), pages 531-534 (1985). The rotation here now only compensates for the effect of the oblique incidence of the molecular beam onto the substrate wafer, so that a noticeable improvement in the homogeneity thereof as compared to the first method is fundamentally achieved.
A French patent application 2490250 discloses a molecular beam epitaxy system wherein conical crucibles as shown in the second figure thereof are used. The molecular beam is obliquely directed onto the substrate and the aperture angle of the crucible is selected of such a size that the entire substrate is swept by the molecular beam.
In Japanese published application JP 6191094 is likewise disclosed a conical crucible having an aperture angle that provides for an adequately large expansion of the molecular beam.
In a Japanese published application JP 63-282190 is disclosed a conical crucible that is circular in a section normal to the longitudinal axis but having an opening cut at an angle to the axis so that the opening is therefore elliptical. This shaping relates to the integration of the crucible into the molecular beam system and does not relate to the cross-sectional shape of the emerging molecular beam.
A Japanese patent application JP 63-252996 discloses a flattened crucible which is obliquely cut, thereby providing an oblong, ellipse-like opening. This opening is intended to enable a large-area, uniform coating of the substrate.
The second method achieves greater uniformity of thickness over a larger substrate region than the afore-mentioned method. However, to achieve a beam flow distribution as in the second method in systems that are optimized according to the first concept, the aperture angle of the conical crucible can only be fundamentally made so large that the entire substrate is situated in the region of the direct molecular beam. This, however, has the following disadvantages:
1. The usable crucible volume decreases noticeably at a given crucible orifice.
2. The growth rates are extremely dependent on the filling of the crucible.
3. A large percentage of the material bypasses the substrate wafer.
4. A large part of the heat radiation also bypasses the substrate and heats up the growth chamber at undesired locations.
5. The cross section of the molecular beam in the plane of the substrate is nearly elliptical, so that the material flow is not fully utilized given standard circular substrates and substrate carriers.